The present invention relates to a sheet metal interstage casing for a pump and a method of manufacturing the same, and more particularly to a sheet metal interstage casing which is pressed into shape for use in a multistage centrifugal pump and to a method of manufacturing the above interstage casing.
Conventionally, there is known a sheet metal interstage casing for a pump wherein the casing is formed of sheet metal such as a stainless steel and is manufactured by press work. This type of interstage casing is shown in FIG. 6 of the accompanying drawings. As shown in FIG. 6, the interstate casing is of a cylindrical receptacle-like structure comprising a cylindrical side wall 1 and a bottom wall (or casing end wall) 2 on an end thereof (the lefthand side in FIG. 6) which is connected to a preceding interstage casing. The opposite axial end surfaces, denoted at 3a and 4a, respectively, of the cylindrical receptacle-like structure are formed by machining a bottom end 3 and an open end 4 perpendicularly to the axis of the interstage casing. The bottom end 3 has a radially outer surface 3b, and the open end 4 has a radially inner surface 4b. The radially outer and inner surfaces 3b, 4b of adjacent interstage casings are machined so that they fit one over the other and form a spigot joint. The desired dimensional accuracy of the axial ends 3a , 4a and the spigot joint surfaces 3b, 4b is maintained by the machining process described above.
The interstage casing houses a guide vane 6 surrounding an impeller 5. The interstage casing has a return passage 8 defined laterally of the guide vane 6 and between the guide vane 6 and a guide vane side wall 7 that is welded to the cylindrical receptacle-like structure of a next adjacent interstage casing. The interstage casing also accommodates a shaft 9 on which the impeller 5 is mounted. A liner ring 10 is attached to the bottom wall 2 and positioned between the inner circumferential surface thereof and the impeller 5.
FIG. 7 of the accompanying drawings shows in fragmentary cross section a vertical-shaft multistage centrifugal pump comprising interstage casings, each having the structure shown in FIG. 6. The interstage casings, each denoted at 1a in FIG. 7, are assembled within an outer casing 11 having a suction port 12 and a discharge port 13. The shaft 9 is rotatably supported in the outer casing 11 through a shaft seal device 14.
When the multistage centrifugal pump is in operation, the liquid to be pumped is drawn from the suction port 12 and pressurized by the successive impellers 5 in the interstage casings 1a. The pressure head of the liquid is restored as the liquid passes through each of the guide vanes 6 and the return passages 8. Finally, the liquid is discharged out of the pump through the discharge port 13.
The interstage casing of pressed sheet metal shown in FIG. 6 maintains a desired degree of dimensional accuracy for diameters and heights through the machining of the four regions, i.e., the radially outer surface 3b and the axial end surface 3a of the male member of a spigot joint on the bottom end 3, and the radially inner surface 4b and the axial end surface 4a of the female member of a spigot joint on the open end 4. The dimensional accuracy of these machined structures is considerably lower than that of casted structures because the wall thickness of the interstage casing structure is small. When the machined casing structure is removed from a machine tool, the cylindrical shape thereof tends to be deformed into an elliptical shape with a dimensional error ranging from 0.3 to 0.5 mm. Since the machined regions are reduced in thickness, it is necessary that the interstage casing blank be thick enough to provide desired mechanical strength.
Sealing between the interstage casings is achieved by liquid gaskets that are of relatively low reliability since no installation space is available between the interstage casings for O-rings that are widely used in pump casings made by casting. Consequently, the interstage casings with liquid gaskets are not suitable for use in applications that require the development of very high pressures or environments that should be kept free from the leakage of the liquid from the pump.